linux-kernel - Re: [PATCH] can: mcp251xfd: silence clang's -Wunaligned-access warning

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Message-ID: <20220518115820.qzvyrnwsb4wvayq2@pengutronix.de>
Date:   Wed, 18 May 2022 13:58:20 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        linux-can@...r.kernel.org
Subject: Re: [PATCH] can: mcp251xfd: silence clang's -Wunaligned-access
 warning

On 18.05.2022 20:43:57, Vincent Mailhol wrote:
> clang emits a -Wunaligned-access warning on union
> mcp251xfd_tx_ojb_load_buf.
> 
> The reason is that field hw_tx_obj (not declared as packed) is being
> packed right after a 16 bits field inside a packed struct:
> 
> | union mcp251xfd_tx_obj_load_buf {
> | 	struct __packed {
> | 		struct mcp251xfd_buf_cmd cmd;
> | 		  /* ^ 16 bits fields */
> | 		struct mcp251xfd_hw_tx_obj_raw hw_tx_obj;
> | 		  /* ^ not declared as packed */
> | 	} nocrc;
> | 	struct __packed {
> | 		struct mcp251xfd_buf_cmd_crc cmd;
> | 		struct mcp251xfd_hw_tx_obj_raw hw_tx_obj;
> | 		__be16 crc;
> | 	} crc;
> | } ____cacheline_aligned;
> 
> Starting from LLVM 14, having an unpacked struct nested in a packed
> struct triggers a warning. c.f. [1].
> 
> This is a false positive because the field is always being accessed
> with the relevant put_unaligned_*() function. Adding __packed to the
> structure declaration silences the warning.
> 
> [1] https://github.com/llvm/llvm-project/issues/55520
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Thank! Applies to linux-can-next/testing.

regards,
Marc

-- 
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